Added deltaX and deltaY properties to FlxMouse.

[EliteMasterEric]

This function makes it easy to compare how the mouse has moved since the last update(). This makes use of the internal _prevX and _prevY values, previously private and inaccessible to the user.

Behaving somewhat similarly to wheel, these properties provide the following:

• diffX: The distance the mouse has moved horizontally since the previous update.
• diffY: The distance the mouse has moved vertically since the previous update.
• diffScreenX: The distance the mouse has moved in screen space horizontally since the previous update.
• diffScreenY: The distance the mouse has moved in screen space vertically since the last update.

This pull request is a rewrite of #2644, which was abandoned due to a faulty Git rebase.

[Geokureli]

x, y, screenX and screenY are defined in FlxPointer which FlxMouse extends, might as well add these properties to FlxPointer instead of FlxMouse

[Geokureli]

on second thought, I see that _prevX was already defined in FlxMouse rather than FlxPointer, so those would need to be moved to the super class.

this makes me wonder if adding 4 new floats to every pointer, by default, is too much overhead, so for now let's just keep it in FlxMouse and revisit this after I finish a tool I'm working on that may help optimize this